Remove reduntant checks from iterators

[RRozak]

Checking if container_ is not null isn't necessary after the iterator is initialized. This PR removes these checks from hasNext methods of iterators of Map, UnorderedMap, UnorderedSet. It also refactors a little. I measured the performance gain on grt phase on black_parrot design:

Without my changes	With my changes
9:38.70	9:31.69
9:38.91	9:31.36
9:38.23	9:32.53
9:38.00	9:30.78
9:30.53	9:35.83
9:22.26	9:27.77
9:23.32	9:13.49
9:20.93	9:13.94
9:20.02	9:13.02
9:21.25	9:13.78

The average gain is slightly less than 1%. It probably speeds up other phases too.

There are still such checks in Set and Vector. When I removed them, I got seg faults. I think that some iterators weren't initialized before the iteration. I haven't tried to fix that yet.